1. Field of the Invention
The present invention relates to a manufacturing method for a variable shape mirror that is mounted on an optical pickup device.
2. Description of Related Art
Conventionally, a compact disc (CD) and a digital versatile disc (DVD) are available in the market as an optical recording medium for recording information such as images and sounds. Further, in recent years, a next-generation DVD is in the stage of being commercialized, which uses a laser beam of violet color having a short wavelength for realizing higher density recording. In order to use such an optical disc, an optical disc apparatus is necessary as a driving apparatus. The optical disc apparatus drives the optical disc to rotate and leads the laser beam onto a recording surface of the optical disc, so that information is recorded or erased and that information is reproduced based on reflection light from the recording surface. Further, in order to realize such a function, the optical disc apparatus is equipped with an optical pickup device as a set of units for emitting the laser beam, leading the laser beam onto the recording surface of the optical disc to form a beam spot, and receiving reflection light from the recording surface of the optical disc.
Here, optical pickup devices, particularly next-generation DVDs are adapted to a high numerical aperture (NA) in accordance with a standard to conform, and influence of the spherical aberration that is generated when a thickness of the optical disc varies slightly becomes conspicuous so that a minute beam spot may not be obtained. Therefore, jitter of a reproduced signal is deteriorated, and recording peak power is lowered, so that recording and reproducing quality is lessened as a result.
In addition, the optical axis of the laser beam may be inclined slightly with respect to the recording surface resulting from warpage or the like of the optical disc. In this case, the optical path of the laser beam is bent so that coma aberration is generated, which makes it difficult to condense the laser beam into an appropriate spot diameter. As a result, recording and reproducing quality is deteriorated. There are other factors of deterioration of recording and reproducing quality, which includes positioning accuracy of an optical system such as an optical lens or a beam splitter that is an element of the optical pickup device, which causes astigmatic aberration.
In order to prevent the above-mentioned situation, there is proposed a variable shape mirror that can correct wave aberration such as the spherical aberration. For example, as shown in FIG. 4, the optical pickup device adopting a variable shape mirror 1 in the optical system is made up of a semiconductor laser 12, a collimator lens 13, a beam splitter 14, the variable shape mirror 1, a quarter wave length plate 15, an objective lens 16, a condensing lens 17, a photo detector 18 and the like. A laser beam emitted from the semiconductor laser 12 is made into parallel rays by the collimator lens 13, passes through the beam splitter 14, is reflected by the variable shape mirror 1 after its polarized state is changed by the quarter wave length plate 15, and is condensed by the objective lens 16 so as to reach the recording surface of the optical disc D. Further, the laser beam reflected by the recording surface of the optical disc D passes through the objective lens 16, is reflected by the variable shape mirror 1, passes through the quarter wave length plate 15, then is reflected by the beam splitter 14 this time, and is condensed by the condensing lens 17 so as to reach the photo detector 18 at last.
The variable shape mirror 1 has a function of a so-called rise up mirror that reflects the laser beam toward the optical disc D and reflects the reflection light from the optical disc D in parallel with the optical disc D. It also has a function of changing its reflection plane if necessary for fine adjustment of a reflection angle of the laser beam, so that wave aberration is corrected. In this case, based on a signal obtained by the photo detector 18, if it is necessary to correct wave aberration, a control portion that is provided to the optical pickup device sends a signal to the variable shape mirror 1 so that a shape of the reflection plane is changed for correcting the aberration.
As to the variable shape mirror 1, there is a type that utilizes characteristics of a piezoelectric element made of a piezoelectric material (see JP-A-2004-109769 and JP-A-2004-226457, for example). This variable shape mirror 1 is generally made up of a support substrate, a mirror substrate that is supported by a support pillar and is opposed to the support substrate, and a piezoelectric element sandwiched between the support substrate and the mirror substrate. The outer surface of the mirror substrate is provided with a reflection film to be a reflection plane for the laser beam. When a predetermined voltage is applied to the piezoelectric element so that an electric field is applied, the piezoelectric element is expanded or contracted between the support substrate and the mirror substrate. The mirror substrate is deformed elastically in accordance with the expansion or the contraction, the reflection film, i.e., the reflection plane is deformed following the deformation of the mirror substrate.
Under the present circumstances, a manufacturing method for such a variable shape mirror that is suitable for mass production is not established yet. For example, in the current situation, in order to manufacture a lot of variable shape mirrors at the same time, there is a method comprising the steps of forming individual parts to be the variable shape mirrors on a wafer, and cutting the wafer for dividing it into individual variable shape mirrors. In this method, individual wafers to be the support substrate and the mirror substrate are prepared, and the support pillar and the piezoelectric element are sandwiched between the wafers within areas to be individual variable shape mirrors. Then, the both wafers and the support pillar, as well as the both wafers and the piezoelectric element are bonded to each other, individually. After that, each of the wafers is cut individually by a disc-shaped dicing saw along boundaries between the areas to be the individual variable shape mirrors, so that it is divided into individual variable shape mirrors. Then, a reflection film is formed on the outer surface of the mirror substrate of each variable shape mirror. Thus, a lot of variable shape mirrors can be obtained at the same time at least.
However, according to this method, distortion will occur in the wafer caused by local residual stresses that are generated at a bonded section between the wafer to be the mirror substrate and the support pillar, and at a bonded section between the wafer and the piezoelectric element. When the wafer becomes the mirror substrate at last, the distortion will remain in the reflection film that is formed on the outer surface of the mirror substrate, so an appropriate reflection plane cannot be obtained. In this case, yield of the obtained variable shape mirrors is reduced, so the method is not suitable for mass production.
In addition, when the wafer is cut by the dicing saw, the wafer, in particular, that is to be the mirror substrate may be damaged as being chipped or broken in many cases. It is because that the wafer is thin and fragile in the first place since the mirror substrate (i.e., its wafer) is required to be deformed elastically in accordance with expansion and contraction of the piezoelectric element as the function of the variable shape mirror described above. Then, yield of the obtained variable shape mirrors is reduced, so the method is still not suitable for mass production.
If the mirror substrate to be each variable shape mirror is cut out from the wafer in advance and the part to be each variable shape mirror is made by using the mirror substrate of a chip level with respect to the support substrate of the wafer level, the damage to the mirror substrate can be reduced not a little when the wafer (for the support substrate) is cut by the dicing saw. In this case, however, the number of mirror substrates of the chip level as elements increases, and management and handle of them are burdensome. Further, manufacturing efficiency of the variable shape mirror is limited, so the method is still not suitable for mass production.